1. Field of the Invention
The subject invention relates to fuel pumps for gas turbine engines, and more particularly, to vane pumps wherein pressurized fluid is supplied to the undervane portion of the vane elements to balance forces imparted thereon.
2. Background of the Related Art
Fixed displacement and variable displacement pumps are used as main fuel pumps in the aviation gas turbine industry. An example of a fixed displacement vane pump is disclosed in U.S. Pat. No. 4,354,809 to Sundberg and a variable displacement vane pump is disclosed in U.S. Pat. No. 5,545,014 to Sundberg et al. The disclosures provided in these patents are herein incorporated by reference to the extent they do not conflict with the present disclosure.
Vane pumps traditionally include a housing, a cam member, a rotor and journal bearings. The housing defines an interior chamber, a fluid inlet and a fluid outlet and the cam member is disposed within the interior chamber of the housing and has a central bore which defines the circumferential boundary of the internal pumping chamber. Mounted for rotational movement within the central bore of the cam member, is a rotor supported by axially opposed journal bearings. Typically, the rotor element has circumferentially spaced apart slots machined therein which support corresponding radially-movable vane elements. The vane elements have a radially outer tip portion which slidably contacts the circumferential portion of the internal pumping chamber and a radially inner undervane portion.
In a single rotation, the vanes of the rotor element of the pump traverse at least four distinct arcuate regions which make up the 360 degree revolution. The first region is the inlet arc segment in which fluid is received into the pumping chamber and over this region the bucket volume increases. The second region is the discharge arc segment in which pressurized fluid is discharged from the pumping chamber and throughout this region, the bucket volume decrease. Lastly, seal arc segments separate the inlet and discharge arc segments and represent the arc segment through which the bucket volume remains substantially constant.
In operation, fluid at a first pressure is fed into the pumping chamber through the housing inlet, and into the space defined between adjacent vane elements, known as the bucket. In positive displacement vane pumps, as the vane elements rotate within the pumping chamber from the inlet region to the outlet region, the configuration of the cam member causes the vanes to retract within the corresponding slots. This causes the volume defined by the bucket to decrease. Since the amount of fluid received into an inlet bucket is greater than that contained within the corresponding discharge bucket, a fluid volume equivalent in size to the volumetric difference is discharged or displaced through the outlet port at a pressure equal to the downstream pressure which must be overcome.
Typically, pumping pressures and velocities are so high within a pump housing that the use of heavy, high wear resistant materials such as tungsten carbide for the vanes and cam member becomes necessary to handle the wear which is caused by these high levels of pressure and velocity.
During this rotation, a radially outward centrifugal force is exerted on the vane elements. At the same time, pressurized fluid within adjacent buckets acts to force the vane elements radially inward. Often, the forces applied to the vanes are not balanced and therefore, the vane tip is either subjected to excessive wear or fluid leaks from within the bucket. This reduces pumping efficiency.
The ideal operating condition for a pump is when the pressure applied to each vane element is balanced and each vane element xe2x80x9cfloatsxe2x80x9d within a corresponding slot in the rotor. This condition results in minimum wear to the vane tips and minimum pressure losses due to the lack of contact between the vane tips and the cam member.
Prior attempts at correcting the unbalanced vane condition have included applying pressure to the undervane portion of the vane. In general, the typical vane pump does not incorporate an undervane pumping feature. Those that do, typically supply pressure from within the buckets in the inlet region to the undervane portion of vanes within the inlet arc. Similarly, the undervane portion of the vanes within the discharge arc are supplied with pressure from the buckets located in the discharge arc. This feature creates a balanced condition within the inlet and discharge arc regions, but does not correct the unbalanced condition in the seal arc regions.
When the vanes are in the first seal arc region, which is located after the inlet arc region and before the discharge arc region, the leading face of the vane is subjected to pressure from the discharge side of the pumping chamber and the trailing face is subjected to pressure from the inlet side of the pumping chamber. Therefore supplying pressure from either the inlet or discharge arc regions will not balance the forces. In fact, an interim pressure equal to half the discharge pressure plus half the inlet pressure is required to balance the forces imparted on the vanes traversing the seal arc regions.
Examples of vane pumps having pressure-balanced vanes adapted to provide undervane pumping are disclosed in U.S. Pat. Nos. 4,354,809 and 5,545,014. The ""809 patent discloses a vane pump incorporating undervane pumping wherein the vanes are hydraulically balanced in not only the inlet and discharge areas but also in the seal arcs. More specifically, the ""809 patent discloses a fixed displacement vane pump which utilizes a series of ports machined in the rotor to supply the pressure to the undervane region. Two ports are provided in the rotor on the leading side of the blade and two ports are provided in the rotor on the trailing side of the blade. All of the ports fluidly communicate with the undervane portion of their associated vane element. Although, this configuration provides a balanced condition, ports having a complex configuration must be machined in the rotor at great expense. Also, in pumps which have a seal arc region with an arc length greater than the arc length between the leading and trailing ports, the pressure supplied to the undervane portion is not a mixture of the pressure from the inlet and discharge arc regions, but rather a mixture of the pressure from the seal arc region and either the discharge or inlet arc regions.
U.S. Pat. No. 5,545,014 to Sundberg et al. teaches a durable, single action, variable displacement vane pump capable of undervane pumping, components thereof and a pressure balancing method which is herein incorporated by reference. The ""014 patent discloses the use of a servo-piston to supply half discharge pressure to the undervane portion of the vane elements when the vanes are positioned in the seal arc region.
In view of the foregoing, a need exists for an improved vane pump which cost effectively balances that forces exerted on each vane element in the inlet arc region, the discharge arc region and the seal arc regions.
The subject application is directed to vane pumps for use with gas turbine engines wherein pressurized fluid is supplied to the undervane portion of the vane elements so as to balance the forces imparted thereon. In a preferred embodiment, the vane pump includes a pump housing, a cam member, a cylindrical rotor member and a chamber. The pump housing has a cylindrical interior chamber formed therein and defines a central axis through which a vertical centerline and a horizontal centerline extend. The cam member is disposed within the interior chamber of the pump housing and has a bore extending therethrough. The bore defines a circumferential surface of a pumping cavity which includes a discharge arc segment, an inlet arc segment and seal arc segments separating the inlet arc segment and the discharge arc segment from one another.
A cylindrical rotor member is mounted for rotational movement within the bore of the cam member, about an axis aligned with the central axis of the interior chamber. The rotor member includes a central body portion which has a plurality of circumferentially spaced apart radially extending vane slots formed therein. Each vane slot supports a corresponding vane element mounted for radial movement therein. Each vane element has a radially outer tip surface adapted for slideably engaging the circumferential surface of the pumping cavity and a radially inner undervane portion within each vane slot.
A chamber is defined within the housing and is positioned for fluid communication with the undervane portion of each vane element and provides a desired pressure thereto. The chamber is in fluid communication with a first pressure source and a second pressure source. The first pressure source is associated with the discharge arc segment of the pumping cavity, and the second pressure source is associated with the inlet arc segment of the pumping cavity.
In a preferred embodiment of the subject invention, the vane pump is a variable displacement vane pump and the cam member is mounted for pivotal movement within the interior chamber of the pump housing about a fulcrum aligned with the vertical centerline of the interior chamber. Alternatively, the vane pump is a fixed displacement vane pump and the cam member is mounted within the pump housing and has a fixed relation with respect to the central axis.
It is envisioned that the circumferential surface of the pump cavity includes an inlet and a discharge arc segment having an arc length of about 150 degrees, and first and second seal arc segments having arc lengths of about 30 degrees However, as would be recognized by those skilled in the art, the arc length of the various segments can vary depending on factors such as the number of inlet and discharge ports and the shape of the circumferential portion of the pumping cavity.
It is further envisioned that in a preferred embodiment of the present invention, the first and second pressure sources are in fluid communication with the chamber each by way of a restrictor. Each restrictor is dimensioned and configured to limit an amount of fluid communicated to the chamber from the first and second pressure sources respectively, thereby creating a desired pressure within the chamber. Also, the chamber is in fluid communication with the undervane portion of each vane element when each vane element passes through the seal arc segments as the rotor member rotates about the central axis.
It is presently preferred that each restrictor is dimensioned and configured to provide a pressure equal to one half of a pressure communicated thereto by the first or second pressure source. In one embodiment, each restrictor includes valve means for selectively controlling the volume of fluid communicated to the chamber by the first and second pressure sources respectively, resulting in the desired pressure within the chamber.
In a preferred embodiment, the vane pump of the present disclosure further includes first and second axially spaced apart end plates which are disposed within the interior chamber of the pump housing. Each end plate has a first surface which is adjacent to the rotor member and forms an axial end portion of the pumping cavity. Each end plate is spaced from the rotor member so as to allow frictionless rotation of the rotor member within the pumping cavity. In this embodiment, the first surface of the first end plate has the chamber and each restrictor is formed therein. Alternatively, and preferably, a chamber and corresponding restrictors can be formed in the first surface of both the first and second end plates. It is also envisioned that first and second channels are formed in the first surface of each end plate. The first channel is configured to provide a path for fluid to communicate from the first pressure source to the restrictor, and the second channel is configured to provide a path for fluid to communicate from the second pressure source to the restrictor.
It is further envisioned that the rotor member can include a plurality of substantially axial fluid passages machined in the central body portion thereof. Each passage is positioned between the plurality of circumferentially spaced apart radial vane slots and provides a path for fluid to communicate axially from the pumping cavity to the first and second end plate.
The present disclosure is also directed to a vane pump which includes a pump housing, a cam member, a cylindrical rotor member and means for providing a pressure to the undervane portions of the vane elements when each vane element rotates through the seal arc segments. Similar to the previously described embodiments, the pump housing has a cylindrical interior chamber which defines a central axis through which a vertical centerline and a horizontal centerline extend. The cam member is disposed within the interior chamber of the pump housing and has a bore extending therethrough. The bore defines a circumferential surface of a pumping cavity which includes a discharge arc segment, an inlet arc segment and seal arc segments separating the inlet arc segment and the discharge arc segment from one another. A cylindrical rotor member is mounted for rotational movement within the bore of the cam member, about an axis aligned with the central axis of the interior chamber. The rotor member includes a central body portion which has a plurality of circumferentially spaced apart radially extending vane slots formed therein, each vane slot supporting a corresponding vane element mounted for radial movement therein.
Unlike the previously described embodiments, this embodiment preferably includes a means for providing a pressure to the undervane portions of the vane elements when each vane element rotates through the seal arc segments. The pressure supplied to the undervane portion of the vane elements is a combination of a first pressure supplied from the discharge arc segment of the pumping cavity and a second pressure supplied from the inlet arc segment of the pumping cavity.
It is presently preferable that the means for providing a pressure to the undervane portions of each vane elements includes a chamber in fluid communication with the first and second pressure sources. Additionally, the first and second pressure sources are each in fluid communication with the chamber each by way of a restrictor. Each restrictor is dimensioned and configured to limit an amount of fluid communicated to the chamber from the first and second pressure sources respectively, thereby creating a desired pressure within the chamber.
The subject application is also directed to a vane pump which includes a pump housing, a cam member, a cylindrical rotor member, first and second axially spaced apart end plates, and first and second pressure chambers.
In a preferred embodiment, the first pressure chamber is formed in the first surface of the first end plate and the second pressure chamber is formed in the first surface of the second end plate. Each chamber is positioned for fluid communication with the undervane portion of each vane element and provides a desired pressure thereto. Each chamber is in fluid communication with a first pressure source and a second pressure source, wherein the first pressure source is associated with the discharge arc segment of the pumping cavity, and the second pressure source is associated with the inlet arc segment of the pumping cavity.
According to the present invention, the pressures acting upon the vanes are balanced so that the vanes are lightly loaded or xe2x80x9cfloatedxe2x80x9d throughout the operation of the present pumps. This reduces wear on the vanes, permits the use of thicker, more durable vanes and, most importantly, provides elasto-hydrodynamic lubrication of the interface of the vane tips and the continuous cam surface. Such balancing is made possible by venting the undervane slot areas to an intermediate fluid pressure in the seal arc segments whereby, as each vane is rotated from the low pressure inlet segment to the high pressure discharge segment, and vice versa, the pressure in the undervane slot areas is automatically regulated to an intermediate pressure at the seal arc segments, whereby the undervane and overvane forces are balanced, which prevents the vane elements from being either urged against the cam surface with excessive force or from losing contact with the cam surface.
The regulation of the undervane pressure permits the use of thicker, more durable vanes by eliminating the unbalanced pressures which are found in the prior art. In the prior art, vanes were made thin to limit the loading of the vane against the cam, because relatively high discharge pressure produces the force that urges the vane tip against the cam, while relatively low inlet pressure acts to relieve the interface pressure between the tip and the cam. The small area of the thin vane allows tolerable loads at the vane tip but often requires dense brittle alloys and results in fragile vanes. Within the inlet arcs of the present invention the undervane areas are subjected to inlet pressure as are the overvane areas. Within the outlet arcs of the pump, the undervane areas are subjected to outlet pressure as are the overvane areas. Within the seal arcs of the pump, the undervane areas are subjected to a pressure that is midway between inlet and discharge pressure, to compensate for the overvane areas which are also subjected half to inlet and half to discharge. More importantly, the regulation of the undervane pressure and xe2x80x9cfloatingxe2x80x9d of the vanes causes the outer surfaces of the vanes to float over the continuous cam surface which is lubricated by the fluid being pumped, whereby metal-to-metal contact and wear are virtually eliminated. This overcomes the need for hard, brittle, wear-resistant, heavy metals, such as tungsten carbide, for the vanes and/or for the cam surface and permits the use of softer, more ductile, lightweight metals.
Those skilled in the art will readily appreciate that the disclosure of the subject application provides an improved vane pump configuration. The features discussed above and other unique features of the vane pump disclosed herein will become more readily apparent from the following description, the accompanying drawings and the appended claims.